Right-Hand Rules have become the de-facto standard.
We pretend that positive charge flows in the direction of the Indicated Current I . Thumb points along this Indicated Current, fingers wrap around the wire (as if to grasp the wire) as the Magnetic Field encircles the Current.

Do this for each part of the loops in a helix (bottom and top ; right and left)
at the center of the helix (on axis),
only the axial component of the magnetic field contributions add up.
The opposite magnetic field is outside the helix, very weak because it is spread out over a cross-section area with radius almost as big as L .